The second External Principle is that only one house, (row, column or box) needs to be considered for each DP digit [within ADP cells], but all [external] occurrences of the ADP digit in that house must be included in the inference analysis.

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Does Principle 2 needed further clarification? If so, please suggest some wording or identify what is missing/confusing.

The one sentence speaks of both internal and external DP digits, but doesn't explicitly mention either one. Perhaps the additions above (in blue) would help.

P.S. You use both the "DP digt" and "ADP digit" terms. I recommend using only the former throughout your post. IOW the pattern is ADP, but the digits are DP.